The present invention relates to an exhaust gas purifying system for an automobile internal combustion engine.
U.S. Pat. No. 3,578,116, patented on May 11, 1971, discloses an exhaust gas purifying system including a selective supply control for interrupting the supply of air-fuel mixture to some of the engine cylinders or combustion chambers during deceleration to improve the combustion conditions so that waste of fuel and emission of a relatively large amount of noxious unburned components of the exhaust gases to the atmosphere can be minimized or substantially eliminated. This conventional system is applicable to a multi-cylinder internal combustion engine of a type having at least two engine cylinders or combustion chambers communicated to a source of air-fuel mixture, which is generally constituted by a carburetor, through an intake manifold of a type including a common duct, having one end communicated to the air-fuel mixture source, and first and second branch ducts extending from the other end of the common duct and leading to the respective engine cylinders.
The selective supply control employed in the above mentioned U.S. patent comprises a shutter or butterfly valve operatively positioned in the first branch duct for selectively interrupting and establishing the fluid circuit between the first branch duct and the corresponding engine cylinder. This shutter valve is normally biased to an opened position to establish the fluid circuit by a biasing spring element, said shutter valve being pivoted in a direction against the biasing spring element to a closed position to interrupt the fluid circuit in response to increase of the negative pressure which would take place inside a portion of the first branch duct upstream of the shutter valve with respect to the direction of flow of the air-fuel mixture towards the corresponding engine cylnder during deceleration of the automobile engine. During the closure of the shutter valve, not only is the air-fuel mixture which would be introduced into the engine cylinder communicated with the first branch duct caused to flow into the second branch duct and then towards the other engine cylinder, but also fresh air is introduced into the first branch duct at a position downstream of the shutter valve in the closed position.
It has been found that, with the conventional system having the construction described above, insufficient reduction of the amount of hydrocarbon and carbon oxide components of the exhaust gases emitted from the automobile engine occurs when the engine is operated under a non-loaded condition (i.e., when the engine torque is zero) and also under a light-loaded condition approximating the non-loaded condition (i.e., when the engine torque takes a positive value but approximating the zero engine torque). The reason for this will now be discussed.
As is well known to those skilled in the art, when the engine is operated under either one of these non-loaded and light-loaded conditions, the exhaust gases then emitted from the automobile engine contain as large an amount of hydrocarbon and carbon oxide components as during deceleration. On the other hand, the closure of the shutter valve when the engine is operated under either of the non-loaded and loaded condition adversely affects the drivability of the engine. Therefore, the shutter valve is operated, i.e., closed, only during the deceleration.
For this reason, with the conventional system now under discussion, a sufficient suppression of the emission of some noxious unburned components of the exhaust gases cannot be achieved under a particular condition during deceleration of the engine. More specifically, the deceleration of the automobile engine is generally understood as including not only a full deceleration wherein the engine is driven while the throttle valve is substantially closed, but also a substantially half-deceleration wherein either the throttle opening is relatively small, or the engine revolutions per unit time, that is, the engine speed, is relatively high, with respect to a characteristic curve the particular engine exhibits during the non-loaded condition in the relationship between the throttle opening and the engine speed, that is, wherein the engine is, even though the throttle valve is opened, driven by an external force, for example, by a moving vehicle body. It is to be noted that, during the half-deceleration of the engine, the engine torque takes a negative value.
In view of the above, the conventional system now under discussion will not operate effectively and satisfactorily unless the shutter valve is controlled at a predetermined timing by the precise detection of whether the engine is operated under the full deceleration or whether the same engine is operated under the half deceleration. This disadvantage appears to be attributed to the utilization of the negative pressure developed in the portion of the first branch duct upstream of the shutter valve. In particular, since the characteristic curve showing the relationship between the throttle opening and the engine speed required to establish an iso-vacuum, that is, an iso-vacuum line, tends to divert during a high speed operating condition of the engine from the characteristic curve the engine exhibits during the non-loaded condition in the same relationship, a relatively large amount of the noxious unburned components of the exhaust gases are emitted during the high speed operating condition of the engine.
U.S. Pat. No. 3,869,858, patented on Mar. 11, 1975, discloses an exhaust gas purifying system including an exhaust manifold having a thermal reactor, a reducing-catalyst unit and an oxidizing-catalyst unit, all fluid-connected in series with each other in the order given above from the automobile internal combustion engine. This exhaust gas purifying system also includes a switching valve assembly for supplying secondary air selectively to respective portions of the exhaust manifold upstream of the thermal reactor and downstream of the reducing-catalyst unit according to a particular engine operating condition. More specifically, in one particular engine operating condition, for example, during the cold-start of the engine, the switching valve is held in position to supply most of the secondary air to the portion of the exhaust manifold upstream of the thermal reactor so that the carbon oxide and hydrocarbon components contained in large quantities in the exhaust gases can be reburned on one hand and, on the other hand, an oxidizing atmosphere can be maintained in a portion of the exhaust manifold between the thermal reactor and the reducing-catalyst unit to enable the reducing catalyst to act like an oxidizing catalyst for further reducing the amounts of the carbon oxide and hydrocarbon components.
In another engine operating condition, for example, after the engine has been warmed up, the switching valve assembly is held in position to supply most of the secondary air to the portion of the exhaust manifild downstream of the reducing-catalyst unit and upstream of the oxidizing-catalyst unit. However, after the engine has been warmed up and when a heavy load is imposed on the engine, the switching valve assembly is, on account of the reduction of negative pressure inside the intake manifold, held in position to supply most of the secondary air to the portion of the exhaust manifold upstream of the thermal reactor so that the thermal reactor serves to reduce the CO and HC proportions in the exhaust gases on one hand and an optimum exhaust gas-air mixing ratio can be provided at the entry of the reducing catalyst unit for the control of the NOx component of the exhaust gases.